Urea is commercially obtained by reacting NH.sub.3 and CO.sub.2 at elevated pressure and temperature to form ammonium carbamate and to simultaneously dehydrate ammonium carbamate to urea. The aqueous urea solution produced in a urea synthesis plant usually contains about one mole of water per mole of urea formed in the urea synthesis reactor. The aqueous urea product solution (generally 75-76 weight percent) is usually processed to a solid anhydrous form by means of the following sequential steps:
1. Practically all of the water contained in the aqueous urea product solution is evaporated to form a more or less pure urea melt; PA1 2. The resulting urea melt at about 270.degree.-300.degree.F. is finely divided into small droplets either by means of a spray head, a spinning conical basket, or a vibrating plate, such devices usually being located at the top of a tall, vertical, cylindrical, rectangular or square tower 100-150 feet high; PA1 3. The molten urea droplets are allowed to fall freely inside the tower countercurrently to an uprising stream of ambient air; PA1 4. The finely divided urea droplets in their free fall inside the tower are cooled by the uprising stream of ambient air and they are frozen into the shape of small spherical particles (prills); PA1 5. The frozen prills are usually cooled further to about 100.degree.-150.degree.F. and are collected at the bottom of the tower.
Alternate techniques can be used to produce molten urea in step 1 described above. One is the crystal-melting technique, which consists of crystallizing the aqueous urea product solution to produce pure crystal urea, which is washed, dried and remelted to produce a substantially pure urea melt.
Referring to the step 5 described above, various methods of collecting solid urea prills at the bottom of the prilling tower have been proposed according to the prior art. In general these methods, mainly listed below, have certain specific disadvantages.
One method consists of collecting the solid urea prills by means of a prilling tower bottom of the shape of a truncated inverted cone or pyramid, with steep, flat sides with an opening at the bottom apex of the inverted cone, through which the prilled urea product is withdrawn. This type of prill collecting bottom has the drawback of frequent solid urea build up on the conical sides and on the bottom section of the inverted cone, with consequent plant shut downs for cleaning operation. Another drawback of this type of prill collecting bottom is the fact that relatively taller and thus more expensive prilling towers are required due to the very steep angle with the horizontal, 60.degree. or more, at which the sides of the inverted cone must be designed in order to facilitate the sliding of the solid prilled urea product towards the bottom opening.
Still another commonly used method consists of collecting the solid prilled urea product by means of a flat horizontal prill tower bottom, which is provided with a collecting rake travelling in a circular motion and thus pushing the solid prilled urea product toward the center of the horizontal prill tower bottom for discharge onto a belt conveyor. Such a prilling tower usually has the drawback of solid urea build up on the prilling tower bottom and on the collecting rake, and the drawback of a relatively higher product degradation due to attrition with a consequent greater air pollution problem in the subsequent solid handling steps.
A third method commonly used in the industry consists of collecting the solid prilled urea product by means of a horizontal fluidized bed of solid urea prills of several inches in depth, which is maintained fluidized by blowing a substantial amount of air through a perforated horizontal metallic surface upwardly into the prilling tower. The excess solid prilled urea product collected in the bottom fluidized bed is overflowed from the fluidized bed over a weir and into a collecting trough. Such a method has the drawback of the instability of the operation due to the collapsing of the fluidized bed of urea prills at the slightest variation in air flow through the perforated horizontal surface. Another drawback of this method is the relatively high electrical power consumption by the large air blowers required to maintain the bed of solid urea prills properly fluidized throughout the full horizontal cross sectional area of the prilling tower bottom section.
For example, in U.S. Pat. No. 3,615,142 (Dahlbom), a prilling tower with an inverted frustum trough bottom exit is described. A louver construction is employed in the trough for the purpose of air being passed through the louvers to cool the prills falling through the tower. This construction has the disadvantage of offering relatively large surface areas of the louvers on which prills can collect. With a louvered construction, there is a substantial build up of prills upon the individual louvers and large dead air areas. With this build up of prills, it is difficult to remove the prills from the bottom section of the tower. With excessive prill build up on the louvers, air flow is seriously reduced; then, hot soft prills fall to the bottom and aggregate and cause physical failure of the louvers. Further, the air is not finely dispersed passing through the louvers.
In U.S. Pat. No. 3,457,336 (Harris), urea or other droplets of molten material are passed through a zone containing a dust bearing gas in order to obtain substantially spherical granules. In all of the arrangements described, a primary source of air is introduced directly into the bottom exit of the system. Secondary, and even tertiary, air sources are employed, particularly to maintain a fluidized bed of dust particles.
With the primary air introduced through the prill exit, there is no control of the falling prills and they tend to build up upon the walls of the tower directly above or in the vicinity of the exit. Then, as the prills stick to the tower walls, they plug the tower completely above the exit and the tower must be taken out of service for cleaning.
A fluidized bed requires a tremendous amount of power for the fluidizing air flow required to maintain such a bed. Fluctuation of air supply will lead to collapse of the bed.
It has been found that by operating a prilling tower bottom according to the process described further below, all the drawbacks which are inherent to the prior art described above are greatly reduced, if not completely eliminated.